The present invention relates to components of radio frequency transmitter monitoring apparatus. More particularly, it relates to a frequency converter for use in a radio frequency transmitter monitoring apparatus which provides the monitoring apparatus with the capability to evaluate transmitter signals without regard to the selected channel. The present means of evaluating these types of transmitted signals require test set adjustment to follow changes of transmitter channel frequency. A need exists for an apparatus which can provide a signal for continuous monitoring of modulation quality without requiring adjustment to follow changes in the transmitter channel.